Crystals having nonlinear optical effects are known as nonlinear optical crystals, wherein the nonlinear optical effects refer to frequency doubling, sum frequency, difference frequency, parameter amplification and other effects. Only crystals without a symmetric center could probably possess nonlinear optical effects. Utilizing the nonlinear optical effects of crystals, the nonlinear optical devices, such as second harmonic generator, up and down frequency converter and optical parametric oscillator, can be manufactured. The laser beam generated by a laser may be subjected to frequency conversion by a nonlinear optical device, thereby obtaining more laser beams with useful wavelength, which leads to wider application of the laser. Based on differences in materials' application wave range, the nonlinear optical crystals are classified into three types: ultraviolet, visible and infrared regions. The nonlinear optical crystals in the visible region are relatively ripe, while the nonlinear optical crystals in ultraviolet and deep ultraviolet regions still have some defects. The transmission cutoff edge and the frequency doubling wavelength of crystals cannot become shorter, which restricts the application of the laser in ultraviolet or deep ultraviolet wavelength regions. Therefore, it has become one of difficult points and future directions in the current study field of the nonlinear optical material to find a new nonlinear optical crystal material with excellent performance in ultraviolet wavelength regions.